Mass spectrograph



Oct. 23, 1951 A. J. DEMPSTER MASS sPEcTRoGRAPH 2 sHEE'rSf-SHEETl 1 Filed Jan. 17, 1947 INVENTOR. yZrZ/ar fezwlvszr BY Oct. 23, 1951 A. J. DEMPSTER 2,572,600

MASS SPECTROGRAPH Filed Jan, 17, 1947 2 SHEETS- SHEET 2 5. lf3'L 30 8 y /00 @rl/d# Patented Oct. 23, 1951 MAss sPEc'rRoGRAPH Arthur J. Dempster, Chicago, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission Application January 17, 1947, Serial No. 722,640

The present invention relates to mass spectrographs and particularly to a mass spectrograph utilizing a variable magnetic lens so as to Vary the angle of incidence of the ion beam upon entering a homogeneous magnetic field.

In a mass spectrograph, the focusing of the ions of different velocities is obtained as a result 'of a geometrical arrangement of the electricand 'magnetic fields, which are so positioned that the dispersion of the ions produced by one field is 'cancelled by the dispersion produced by the other field for a given velocity difference. In actual practice, the ion beam will not be substantially parallel upon entering the magnetic field, but will either be divergent or convergent due to the inherent design defects characteristic in the mechanical construction of the electrostatic deflecting condenser and collimating slit system accommodated in the mass spectrograph. These inherent defects will produce variation in the propagation so as to produce non-parallelism of the ion beam. In order to compensate for the deviation of the ion beam, the present invention comprehends an adjustable magnetic lens to vary the angle of incidence of the ion beam upon entering the magnetic eld.

The principal object ofthe present invention is l to provide an improved mass spectrograph, which will correct for the divergence or convergence of the associated ion beam.

As stated generally above, the object of the: present invention is accomplished by providingl `an adjustable magnetic lens apparatus to afford fa Variable angle of incidence at the entrant boundary to the homogeneous magnetic field, in l contrast with the prior known spectrographs emy 7 Claims. (Cl. 250-41.9)

-These various structural features, and other ob- M jects and advantages of theinvention, will be made more apparent by referenceto the accompanying drawings and the following description of a preferred embodiment thereof. In the drawings; v Fig. 1 is a sectional plan tion, showing the magnetic deecting assembly Icdupled to a posterior portion of the collimating and electrostaticedeilecting unit of a massspe'- vtrograph;

Fig. 2is an enlarged perspectiveview. of the variable magnetic lens structure, a portionthere- Lof being broken away;

l, Fig. 3 is a vertical cross section, partly inele- .Jvation, taken on the line 3,-3of Fig. 1.;

view, partly in eleva-I Fig. 4 is a vertical sectional view, partly in elevation, taken along the line 4 4 of Fig. 1;

Fig. 5 is a schematic diagram showing a parallel incident ion beam being bent through an angle l of by means of a combined variable magnetic posed homogeneous magnetic field; and

Fig. '7 is a schematic diagram showing a convergent entrant ion beam which is deflected and focused at a defined focal point by means cfa combined variable magnetic lens and a homogeneous -magnetic field accommodated in a 'mass spectrograph.

Heretofore various combinations of magnetic fields have been employed in mass spectrographs to separate ion beams with similar, but not idenytical, charge to mass ratios, and to record the .composite mass spectrum. Theoretical interpretations and calculations have been set forth wherein the design of the electrostatic condenser =and the magnetic deflecting system have been compared with a corresponding optical lens and prism presentation. In the actual design and lconstruction of the mass spectrograph,v it was found that the variance in the mechanical construction of the mass spectrograph from the theoretical calculations produced a deviationof the associated ion beam from parallelism so that .the incident ion beam was either divergent Ior convergent upon entering a homogeneous magnetic field. The present invention comprises novel means to compensate for the non-parallelism of the ion beam in order to facilitate simplicity in construction and operation. In addition, it has been found that an ion beam can be made to focus at a predetermined focal position Von va recording means, such as a photographic plate by the varying of the electrostatic ield -developed across the electrostatic condenser or lens. In this manner, the ion beam can be slightly convergent or divergent prior to entering the magnetic field, and the focus of the beam can be ladjusted by adjusting the angle of entrance into the magnetic field by means of a variable magnetic lens apparatus. This procedure is advan- :tageous in operating a mass spectrograph so as to obtain good resolution of the spectrum over a fwell defined mass range, A

, `The novel features embodied herein as shown in Fig. 1 .include a device for varying the angle of incidence of an ion beam entering into a `means of stud bolts A2.

ymately to the height of said aperture 55.

deecting magnetic field, generally designated by numeral 8. The mass spectrograph as fragmentarily shown in Fig. 1 comprises a non-magnetic metallic shell or housing I which contains the electrostatic deflecting condenser designated generally by numeral |I and a collimating slit unit indicated by numeral 20. The collimating slit unit 20 is positioned within the housing I0 and is mounted on an electrical insulating base plate 2| formed, for example, of glass bonded mica sheet rigidly connected to the'housing I0 by means of a plurality of screws 22. Attached to the base plate 2| is a non-magnetic metallic bracket 23, for example, brass, which is ai'ixed to the base plate 2| by means of screws 25.

Mounted perpendicularly to said base plate 2|V and attached to said bracket 23 and mounted by means of screws 25, as s hown in Fig, 3, is a slit holder 26 having the collimating slit 2'I therein shown enlarged for the purpose of clarity in illustration. The slit is preferably rectangular 'with dimensions of approximately 0.9 mm. by 2.4

mm. Anterior to said collimating slit unit 20 and attached to the insulating base plate 2|, is

the conventional electrostatic deflecting condenser comprising condenser plates I3 and I5 Ymounted in spaced relationship.

The shell or housing I0 is attached to a face plate 40 which is made of non-magnetic mate- -rial such as brass. said face plate 40 being mechanically attached to said shell housing I5 by A resilient easketinsert 43, for example. rubber, is adapted to form a vacuum tight seal upon tightening the stud bolts The magnetic deecting unit 50 comprises an upper magnetic pole piece 6I, lower magnetic pole piece 62 and non-magnetic side walls 53 and 54, for example. brass, said side walls 53 and 54 comprising a housing wherein an aperture 55 (Fig. 4) is disposed between the pieces 6| and Said side walls 53 and 5d, are connected as bv soldering, to said face plate 40, as shown in An adjustable magnetic lens unit 30 is recessed in the pieces 6I and 52 at the point of entrance of an ion beam to the aperture 55 of the magnetic deilecting unit 50. The lens 30 comprises spaced magnets 5| and 52, as shown in elevation in Fig. 2. The spaced magnets 5I and 52 are preferably of vsemi-circular cylindrical design', machined of magnetic materials, such as Armco iron, and inter-connected with a non-magnetic strip or member 56 by means of mounting screws The magnets 5| and 52 define a gap therebetween. Said air gap 58 corresponds approxi- At the opposite end surfaces of the spaced magnets 5| and 52 are mounting pivots 59 which are inserted into complementary sockets 63 and 04 in the upper and lower pole pieces 6I and 62 respectively, as shown in Figs. 2 and 3. Said recesses 63 and 64 afford a housing, or cavity, wherein the rotatable magnetic lens 30 is disposed. This manner of mounting allows for rotation of the variable magnetic lens 30 by the in- Ward or outward motion of an actuating rod 66 `which is pivotally connected to a pin 61 mounted on the upper surface of the lower magnet 52 near the outer periphery thereof, as shown in Fig. 1 and 2. A magnetic shutter 6B, preferably made of non-magnetic metallic material, for example, brass, is mounted on an outer periphery of the magnetic lens 30 as shown in Figs. 1 2

and 4; and upon rotating said lens 30 approximately 90 from its normal position where the at surfaces of the lens 30 are approximately perpendicular to the incident ion beam as illustrated in Figs. 1 and 4, said shutter 68 will effectively cut off the incident ion beam.

The rod 66 extends through an opening 'I0 in the non-magnetic sidewall 53 through a nonmagnetic tubular member which is rigidly attached to an annular base plate I2 as by means of solder. A exible bellows 16 is connected at one end thereof to the plate I2 as by soldering, and the opposite end of said bellows I6 is soldered to an annular disc which is soldered to the extended rod 56 so as to form a flexible, moveable vacuum tight seal with respect to the mass spectrographic device. The rod 55 is preferably provided with a knob 8|)k for convenient actuation thereof.

A photographic recording assembly 99 for recording the impinging ion beam comprises a photographic plate I 00, and a photographic plate holder |02 which has an opening communicating with the aperture 55 in order that the impinging ion beam will strike photographic emulsiiied surfaces of said plate |00. At the posterior end of said magnetic deflecting unit 50, is a tail assembly indicated by numeral |04 comprising side walls |05 and |06 so as to forma vacuumchamber |01 wherein the posterior portion of said photographic plate |00 is disposed. Attached to the posterior ends of said side walls |05 and |06 are L-shaped brackets |08 connected to I.- shaped brackets I|| on the end plate 3 byl bolts |09. A resilient gasket IIA is disposed intermediate between said end plate I|3 and said side walls |05 and |06 so as to afford a vacuum tight seal. Upon disassembl-ing said stud bolts |09, the photographic plate |00 is easily accessible and can be inserted or removed through said chamber |01.

A number of focusing schemes have been titi lized in mass spectrographs, some resulting iii vvelocity focusing, some designed in the direction of focusing a non-collimated beam, and other' constructions aimed more directly inachieving a linear mass range. These instruments pro# viding double focusing of the ion beam, by using' consecutive electrostatic and magnetic focusing of a well collimated ion beam, have provided ie-f sults so as to record the ion spectrum on a photographic plate. Generally, the mass scale is not linear and must be determined from calculations on the geometry of the focusing conditions; also the resolution must be determined from the accuracy of collimationl by the associated co1- limating slit 20 and of electrostatic deflection by the electrostatic condenser I| deilecting the beam of ions from the source.

It may be noted that the theory and design' of various spectrographs are subject to constructional variations and that this inherent variation will produce a non-parallel ion beam which may be either convergent or divergent. Since it is extremely difficult to adjust the critical interrela- Vtionship and alignment of the associated collimating slit 20 and the electrostatic deflecting condenser I|, it has been found that the ion vbeam is generally divergent or convergent upon plate IUI) at a focal point IIU. The propagation of a parallel ion beam I I I is generally considered desirable in operating a mass spectrograph. When an associated ion beam I I I is divergent, as shown in Fig. 6 after leaving the collimating and electrostatic deflecting assembly, a, flat magnetic lens face II3 of the magnetic lens 30 is rotated so that the angle of incidence of the ion beam entering the magnetic field 8 is substantially varied in order to compensate for the divergency of the ion beam III so as to focus said beam III on the corresponding focal point IIO. When an associated ion beam III, is slightly convergent as shown in Fig. '7, the interface I I3 of the magnetic lens 30 is rotated as shown, and the ion beam III is again focused on the focal point II'0.

In addition, it has been demonstrated that by varying the electrostatic eld developed across the electrostatic condenser II, the ion beam III can be made slightly divergent or convergent so that the direction of said beam I I I prior to entering the homogeneous magnetic eld 8, can be adjusted by the magnetic lens 30 to focus at a predetermined focal point IIIJ. In this manner of operation, the ion beam I II may be purposely made slightly convergent or divergent to accommodate a predetermined focal point of' the ion beam II'I by adjusting the angle of entrance of said beam III into the magnetic field 8 by the variable magnetic lens 3l).

It will be apparent that modications will be readily suggested to others skilled in the art as the result of the teachings of my invention, hence, it should be understood that my invention is not limited by specific construction hereinbefore described, which is an exemplary embodiment, and should be restricted only insofar as set forth in the following claims.

What is claimed is:

1. A magnetic lens comprising spaced magnets arranged end-to-end defining an ion path therebetween, rigid non-magnetic means interconnecting said magnets, pivots on the remote sides of said magnets, disposed on a common axis, and means to rotate the magnets on the pivots attached to one of the magnets.

2. A magnetic lens comprising spaced magnets arranged end-to-end defining an ion path therebetween, rigid non-magnetic means interconnecting said magnets, pivots on the remote sides of said magnets disposed on a common axis extending through the magnets, a rod attached to one of the magnets at a point other than the axis, said rod being adapted to extend to an actuating means, and a shutter mounted on the magnets and extending between the magnets parallel to the axis.

3. A magnetic lens unit comprising spaced magnets in the form of semi-cylinders, arranged end to end, rigid means for interconnecting said magnets, pivots on the remote sides of the respective magnets disposed in a common axis vcoaxial to said cylinders. and means on the end of one of said magnets adjacent the other for connecting the unit to associated actuating means for rotating the same.

4. A magnetic lens unit comprising spaced magnets in the form of semi-cylinders, arranged end to end, rigid means for interconnecting said magnets, pivots on the remote sides of the respective magnets disposed in a common axis coaxial to said cylinders, and means on the end of one of said magnets adjacent the other for connecting the unit to associated actuating means for rotating the same, and a shutter extending across the space between said magnets at one side of said axis and secured to said unit.

5. A magnetic lens unit comprising spacedmagnets in the form of semi-cylinders arranged end to end, rigid means for interconnecting said magnets, pivots on the remote sides of the respective magnets disposed in a common axis coaxial to said cylinders, and means on the end of one of said magnets adjacent the other for connecting the unit to associated actuating means for rotating the same, and a non-magnetic shutter having a portion extending between the magnets and secured to at least one thereof, said shutter extending across the space between said magnets at one side of said axis.

6. In a mass spectrograph, an aperture, a magnetic field therein for deflecting an associated ion beam, means communicating with said aperture for recording an ion spectrum, and magnetic lens means for compensating for the divergence or `convergence of the paths of the ions in said beams, said lens means comprising spaced interconnected magnets defining a, gap aligned with said aperture, means for rotating said magnets, and shutter means extending across the gap at one side thereof for closing said aperture against flow of ions therethrough in one rotatable position of said magnets.

7. In a mass spectrograph, an aperture, a magnetic field therein for deflecting an associated ion beam, means rcommunicating with said aperture for recording an ion spectrum, and magnetic lens means for compensating for the divergence or convergence of the paths of the ions in said beam, said lens means comprising spaced interconnected rotatable magnets in the form of semi-cylinders arranged end to end.

ARTHUR J. DEMPSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,200,039 Nicoll May 7, 1940 2,405,306 Hillier et al Aug. 6, 1946 OTHER REFERENCES Bleakney, Article in American Physics Teacher, February 1936, volume 4, pages 12-17. 

